Conventionally, wireless base station devices (hereinafter, often referred to simply as the “base station”) are provided with an amplifying unit that amplifies the power of a transmission signal. In general, in the base station, the amplifying unit is operated near the saturation region of the amplifying unit in order to enhance the power efficiency of the amplifying unit. However, when the amplifying unit is operated near the saturation region, non-linear distortion increases. Therefore, in order to suppress this non-linear distortion and reduce the adjacent channel leakage ratio (ACLR), the base station is provided with a distortion compensating unit that compensates for the non-linear distortion.
As one of distortion compensation systems used in the distortion compensating unit, there is a “predistortion (hereinafter, often referred to as the “PD”) system.” The distortion compensating unit of the PD system enhances the linearity of the output of the amplifying unit and suppresses the distortion of the output of the amplifying unit by multiplying a transmission baseband signal before input to the amplifying unit by a distortion compensation coefficient having the inverse characteristic of the non-linear distortion of the amplifying unit in advance. The signal resulting from the multiplication of the transmission baseband signal by the distortion compensation coefficient is often referred to as the “PD signal.” Thus, the PD signal is a signal that gets distorted in advance before input to the amplifying unit in accordance with the inverse characteristic of the non-linear distortion of the amplifying unit.
For example, as the distortion compensating unit of the PD system, there is one that includes a lookup table (LUT) in which plural distortion compensation coefficients are stored and specifies, to the LUT, an address according to the power of the transmission baseband signal to read out the distortion compensation coefficient from the LUT. The distortion compensation coefficients stored in the LUT are sequentially updated so that the error between the transmission baseband signal as a reference signal and a signal that is output from the amplifying unit and is fed back (hereinafter, often referred to as the “feedback signal”), obtained by comparing both signals, may be minimized. As a related-art document, there is Japanese Laid-open Patent Publication No. 2007-96775.